How bridges get built · Precast concrete decks · Span-by-span precast segmental with gantry
Also called: span-by-span erection, overhead gantry, underslung gantry, segmental viaduct
How it is done
- Set up the casting yard so it runs two or three spans ahead of erection, and assemble, commission and load-test the gantry on the first span.
- Deliver segments in erection order by road below the gantry (lifted by its winch) or along the completed deck.
- Hang all segments of a span from the overhead gantry, or place them on the underslung trusses, with the pier segments set on shims or temporary supports.
- Align the segments to the survey, apply epoxy to the match-cast faces and close each joint with temporary bars to about 0.3 MPa average compression.
- Where the design uses them, cast narrow closure joints at the piers to absorb tolerances; install and stress the span tendons, often external through deviators.
- Lower the span onto its bearings, release it from the gantry and launch the gantry to the next span after a launch checklist (hold point).
- Grout or otherwise protect the tendons and seal the anchorages behind the erection front.
Key numbers
- Span30–50 m, economic about 35–45 m
- Segment length2.5–3.5 m; weight typically 35–80 t
- Cycleone span per 2–4 days per gantry
- Temporary compression across epoxy jointsabout 0.3 MPa average
- Gantry80–120 m long, 300–600 t
- Break-evenroughly 15–20 spans or more
Where it fits
- Viaducts with 30-50 m spans and more than 15-20 spans, urban and marine settings, metros, tight sites with only overhead access.
Where it does not
- Few spans, varying spans, spans over 50-55 m (weight), sharp curves for overhead gantries.
Choosing it
- Choose it over balanced cantilever for 30–50 m spans in large numbers: faster, constant depth and no unbalanced moment.
- Choose it over full-span precast when there are fewer than a few hundred spans, the route cannot carry 600–1,000 t spans, or curves are tight (urban metros).
- Choose it over an MSS on long viaducts of several kilometres, where the casting yard pays for itself and speed matters.
- Prefer an underslung gantry when headroom above the deck is limited; prefer an overhead gantry when pier heads cannot carry brackets or segments are delivered from below.
Plant, pace and money
PlantGantry 80-120 m, 300-600 t; casting yard with short-line cells; segment transport.
ProductivityOne span per two to four days per gantry.
CostDeck USD 1,300-1,900 per m2 including yard and gantry over a long viaduct.
Risks and controls
What goes wrong
- Gantry collapse during launching (several accidents), epoxy joint failure, geometry error accumulating across segments, temporary bar failure.
Quality assurance
Match-cast geometry control in the yard, epoxy application in temperature limits, stressing records, gantry launch procedure.
Origins
Span-by-span erection of precast segments with external tendons was introduced on the Long Key Bridge in the Florida Keys (1980), designed by Figg and Muller, and became the standard for long urban, metro and marine viaducts.
Examples
Long Key and Seven Mile bridges (Florida Keys), Bang Na Expressway (Bangkok), Dubai Metro, and many metro and expressway viaducts in Asia and the Gulf.
Case studies
Long Key Bridge, Florida KeysUSA · 1980The first span-by-span precast segmental bridge in the US, with external tendons and spans of about 36 m.
Bang Na Expressway, BangkokThailand · 2000About 55 km of elevated expressway built from precast segments erected span by span with overhead gantries, one of the longest bridges in the world at completion.
Dubai Metro Red and Green LinesUAE · 2009–2011Elevated viaducts built largely from precast segments erected span by span by launching gantries working above live streets.
In the bridge game
What Bridge Master applies when you pick it
Deck types: Precast segmental box, PSC box (cast in place), PSC I-girder, PSC U-girder, Voided slab. Spans 30-50 m, up to 60 m.
- +35: Kilometres of 30-55 m spans: gantry span-by-span.
- +15: Marine or urban viaduct with no ground access.
- -40: Gantry and yard cannot be amortised.
- -40: Beyond span-by-span.
- -20: Gantry geometry on tight curves is awkward.
- not allowed: Span-by-span gantry beyond feasibility.
Related methods
Balanced cantilever with precast segmentsSegment casting: short-line and long-line match castingMovable scaffolding system (MSS)Launching girder (beam launcher) for precast girdersProgressive placement of precast segments
Further reading
ASBI Construction Practices Handbook · AASHTO Guide Specifications for Segmental Bridges